Low-cost fast variable optical attenuator for optical wavelength tracking

ABSTRACT

Variable optical attenuator (VOA) formed by disposing upon a substrate a waveguide, a p-type region and an n-type region about the waveguide, and an epi-silicon region disposed upon the waveguide, the VOA responsive to a bias current to controllably inject carriers into the waveguide to attenuate thereby optical signal propagating through the waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/639,525, filed Dec. 16, 2009 now U.S. Pat. No. 8,326,109, entitledLOW-COST FAST VARIABLE OPTICAL ATTENUATOR FOR OPTICAL WAVELENGTHTRACKING, which application is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to optical communications and, more particularly,to variable optical attenuators (VOAs).

BACKGROUND

Transparent optical communications systems use variable opticalattenuators (VOA) for each wavelength channel to impose a relatively lowfrequency dither tone which is below the data channel in frequency andcan be used to identify channels and their power levels in transmission.This VOA must be both sufficiently fast (e.g., 1 MHz), physically smalland low cost, especially for applications with over 50 wavelengths ineach system.

Some optical communications systems use bulk-silicon ridge waveguideVOAs, which are fast enough in attenuation response and relativelysmall. However, silicon waveguide VOAs use conventional fibre v-grooveblocks to attach fibres on each side. These fibres are both large andcostly relative to the size and cost of the silicon waveguide VOA chip.For reference, for optical splitters and arrayed-waveguide gratingfilters (both of which are high volume integrated waveguide devices),the integrated optical die tends to be less expensive than the fibreassembly used to attached the die to the fibres.

Another issue with silicon waveguide VOAs the use of a low-contrastwaveguide design, which increases both the chip area (leading to lowerwafer yield and higher cost) and requires fibre to be attached to bothsides of the VOA (which increases the overall size of the component).

SUMMARY

Various deficiencies of the prior art are addressed by a variableoptical attenuator (VOA) formed by disposing upon a substrate awaveguide, a p-type region and an n-type region about the waveguide, andan epi-silicon region disposed upon the waveguide, the VOA responsive toa bias current to controllably inject carriers into the waveguide toattenuate thereby optical signal propagating through the waveguide.

One embodiment is an apparatus comprising a substrate, having disposedthereon a waveguide; an epi-silicon region disposed upon the waveguide;and a variable optical attenuator (VOA) comprising a p-type region andan n-type region, the VOA disposed about the waveguide and responsive toa bias current to controllably inject carriers into the waveguide toattenuate thereby optical signal propagating through the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a side view of an optical waveguide apparatus accordingto one embodiment;

FIG. 2 depicts a cross sectional view of an optical waveguide accordingto one embodiment;

FIG. 3 depicts an optical waveguide apparatus according to oneembodiment;

FIG. 4 depicts a side view of a VOA chip suitable for use in the opticalwaveguide apparatus of FIG. 3

FIG. 5 depicts an optical waveguide apparatus according to anotherembodiment;

FIG. 6 depicts an avalanche photodiode (APD) receiver apparatusaccording to one embodiment; and

FIG. 7 depicts an optical waveguide apparatus according to anotherembodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

Various embodiments will be discussed within the context of a highlyconfined silicon waveguide design including one or more integratedcoupling lenses, the design enabling a significant reduction in overallpackaging costs and size, as well as a higher wafer yield and so furthercost reduction.

Various embodiments provide a tunable optical phase shifter thatsupports both TE and TM polarizations i.e., exhibits polarizationinsensitivity.

FIG. 1 depicts a side view of an optical waveguide apparatus accordingto one embodiment. Specifically, the apparatus comprises a substratesuch as a silicon-on-insulator wafer (not shown) supports or otherwisehas disposed upon it a waveguide of a standard thickness, illustratively220 nm. It will be appreciated by those skilled in the art that otherwaveguide thicknesses may be used within the context of the presentinvention. For example, 100 nm waveguides are presently used in variousapplications. Theoretical limits for silicon waveguides indicate thatthicknesses down to approximately 10 nm may also be employed, and suchsmall geometry waveguides are also contemplated in various embodiments.

Referring to FIG. 1, an input optical signal IN is received by thewaveguide at an input port (left side of figure) and propagated throughthe waveguide to provide an output optical signal OUT at an output port(right side of figure). The optical signal propagating through thewaveguide is controllably attenuated via a P-I-N injection region (i.e.,a P-I-N diode). The P-I-N injection region comprises regions on eitherside of the waveguide that are doped p-type and n-type to enablecarriers to be injected into the waveguide. These carriers induceoptical absorption, and hence, attenuation based on the bias currentapplied to the P-I-N diode structure. The P-I-N injection region isdepicted as having a thickness of 340 nm. In one embodiment, the P-I-Ninjection region is epitaxially grown. In other embodiments, the P-I-Ninjection region is deposited as a poly-silicon layer.

To reduce optical losses due to modal mismatch between the thin siliconwaveguide (˜220 nm) and the P-I-N injection region (˜340 nm), anoptional additional waveguide segment having an intermediate thicknessis added at both ends of the P-I-N injection region. The optionalintermediate thickness waveguide segments act like semi-adiabatic tapersthat help reducing optical transition losses between the varioussections of the VOA. Specifically, first and second vertical step indexregions having a thickness of 280 nm are optionally provided between theP-I-N injection region and, respectively, waveguide input and waveguideoutput. In various embodiments, the length of the index or taperingregions ranges from approximately 5 um to approximately 200 um. Largeror smaller index/tapering regions may be used depending upon theselected geometry and other considerations known to those skilled in theart and informed by the teachings provided herein. In some embodiments,multiple vertical steps are provided within the index/tapering regions.Generally speaking, the geometry, length, number of steps and so on isselected to reduce optical coupling losses between the waveguide (e.g.,shown here as 220 nm) and the thicker PIN region (e.g., shown here as340 nm).

One embodiment of the optical waveguide apparatus described above withrespect to FIG. 1 comprises a highly confined optical waveguide made ona silicon-on-insulator wafer or substrate. The waveguide dimensions inone embodiment are approximately 550 nm wide and 220 nm thick, whichprovides for tight bending of the waveguide and minimization of the chipsize and consequent cost. As previously noted, waveguides havingdifferent thicknesses may be used within the context of the presentinvention. In embodiments using different thickness waveguides, thethickness of the vertical step index regions and/or P-I-N injectionregion are adapted as would be known to those skilled in the art andinformed by the teachings of the present specification. In variousembodiments, the wafer or substrate may comprise an Indium-Phosphatesubstrate, a Lithium Niobate substrate or other substrate.

High optical confinement in narrow waveguides may result in a large VOApolarization dependence loss. This loss is acceptable for manyapplications where a VOA is used, such as, illustratively, at the outputof a laser where the injected light is single polarization. However, fora more widely useful device the inventors have provided a substantiallypolarization independent design by adjusting the guiding region geometryat the P-I-N carrier injection region such that the modal profiles ofthe TE/TM polarization components are substantially identical.Specifically, a symmetrical nearly symmetrical waveguide geometry acrossthe P-I-N junction is provided by, illustratively, epitaxially growingsilicon on top of the P-I-N injection region.

FIG. 2 depicts a cross sectional view of an optical waveguide accordingto one embodiment. Specifically, FIG. 2 depicts a cross sectional viewof an optical waveguide such as discussed above with respect to FIG. 1in which an additional 160 nm of epitaxially grown silicon (epi-silicon)is added to the guiding region which, illustratively, has a preselectedwidth of 340 nm.

In various embodiments the epi-silicon region is formed as a layerdisposed upon the P-I-N injection region or formed as part of the P-I-Ninjection region including the VOA. That is, the P-I-N injection regionmay be adapted to include the epi-silicon region. The transition regionhas a thickness that is intermediate the maximum and standardthicknesses. Referring to FIG. 1, a transition region having a thicknessof 280 nm is depicted. This geometry is selected in this embodimentbecause 280 nm is between the 220 nm circuit photonic geometry of thewaveguide and the 340 nm P-I-N injection region geometry.

In the case of thinner photonic geometries, the various embodiments aremodified to reflect correspondingly thinner P-I-N injection region andepi-silicon (transition) region geometries. For example, if thewaveguide is formed using a 110 nm geometry, the P-I-N injection regionis formed using, illustratively, a 170 nm geometry and the epi-silicon(transition) region is formed using a 140 nm geometry. While thegeometry in this embodiment reflects a 50% scaling of the FIG. 1geometries, it will be appreciated by those skilled in the art andinformed by the present disclosure that a simple scaling of geometrywill not always be appropriate due to various physical and practicallimitations, especially as photonic geometry approaches 10 nm.

FIG. 3 depicts an optical waveguide apparatus according to oneembodiment. Specifically, FIG. 3 depicts an optical waveguide apparatus300 comprising a VOA chip 310, a VOA controller 320 and an optionaloff-chip bias circuit 330.

The VOA chip 310 comprises a substrate 312 having disposed thereon awaveguide 314, a VOA circuit or structure 316, an optional on-chip biascircuit 318, and trench structures adapted to receive an input ball lensBL-1 and an output ball lens BL-2. Advantageously, the use of a balllens within the context of the present embodiment provides excellentcoupling to connecting optical media, such as optical fibre, fibreopticor silicon nanowires, lightpipes and so on. For example, coupling lossbetween an optical chip and a single mode optical fibre is about 3 to 4dB using standard coupling techniques. It has been determined thatcoupling loss between an optical chip and a single mode optical fibre isabout 0.5 to 1 dB using a ball lens.

The VOA chip 310 receives an input optical signal IN via the input balllens BL-1. In operation, the input ball lens BL-1 is in opticalcommunication with an optical fibre or other medium conveying the inputoptical signal IN. The input optical signal IN is conveyed by the inputball lens BL-1 to a first or proximate portion of the waveguide 314. Thewaveguide 314 propagates the optical signal from the input ball lensBL-1 two an output ball lens BL-2. The output ball lens BL-2 providesthe propagated optical signal as an output optical signal OUT. Inoperation, the output ball lens BL-2 is in optical communication with anoptical fibre or other medium conveying the output optical signal OUT.

The waveguide 314 is disposed between a p-type region 316+ and an n-typeregion 316− of the VOA 316. The VOA 316, in response to a bias currentsignal BC, operates to controllably inject carriers into the waveguide314 such that attenuation of the optical signal passing through thewaveguide 314 is achieved.

In one embodiment, the VOA chip 310 includes an on-chip bias circuit 318that produces the bias current signal BC in response to a control signalreceived from the VOA controller 320. An alternate embodiment, the VOAchip 310 does not include an on-chip bias circuit 318. Rather, anexternal bias circuit 330 is used to provide the bias current signal BCto the VOA 316.

The VOA controller 320 provides the control signal to the bias circuit318/330 in response to a feedback signal received from, illustratively,and optical power measuring/detection device (not shown). The VOAcontroller 320 operates to control the optical intensity within thewaveguide such that the optical intensity is within a desired range,held constant, constrained from violating a high-power threshold and soon.

FIG. 4 depicts a side view of a VOA chip suitable for use in the opticalwaveguide apparatus of FIG. 3. Specifically, FIG. 4 depicts a VOA chip410 similar to the VOA chip 310 described above with respect to FIG. 3.Referring to FIG. 4, visible elements of the VOA chip 410 comprise asubstrate 412, a waveguide 414, first ball lens BL-1 and second balllens BL-2. It is also noted that the substrate 412 is formed with afirst trench T1 adapted to receive first ball lens BL-1 and a secondtrench T2 to adapted to receive second ball lens BL-2. Variousembodiments of the VOA chip depicted in FIG. 4 may be manufactured as anapproximately 1 mm by 5 mm chip.

In various embodiments, the wafer or substrate further includes one ormore power taps, such as integrated waveguide directional couplers orwavelength insensitive cascaded directional couplers adapted to sampleor divert, illustratively, 2% to 5% of the optical power. In variousembodiments, other technologies may be used (e.g., partially silveredmirrors and the like) and/or other amounts of optical power passingthrough the waveguide may be diverted (e.g., below 2% or above 5%).These embodiments also include one or more optical power detectors, suchas PIN diode or integrated germanium or silicon germanium PIN detectorsor other detector technologies adapted to detect an optical power levelassociated with the sampled or diverted optical signal portion(s). Forexample, waveguide power taps and detectors may be disposed on thesubstrate at the input and/or output of the VOA to provide feedbacksignals adapted for use by a controller to set VOA attenuation levels.

By using a high contrast waveguide design, the inventors have provided awaveguide circuit that may be “folded” so that a single imaging balllens can be used to inject light into an input of a VOA chip orassembly, and also collect light emitted from an output of the VOA chipor assembly. In this manner, the very compact and low cost VOA design isprovided which may be used within the context of a circuit pack or otherapparatus. In another embodiment, two ball lenses are used on a singleside of a chip or assembly for a package that has both input and outputfibres on the same facet of the package, which embodiment is preferredfor certain face-plate mounting configurations. The ball lens providesseveral advantages; namely, it is lower cost than a fibre v-grooveassembly and it can provide improved optical coupling to various fibres.In various embodiments of the invention, optical input/output isprovided by, illustratively, on chip mode converters, adiabatic tapersand the like.

FIG. 5 depicts an optical waveguide apparatus according to anotherembodiment. Specifically, an optical waveguide apparatus 500 comprises aVOA chip 510 and a VOA controller 520. The VOA chip 510 comprises asubstrate 512 including a trench T adapted to receive a ball lens BL. Aninput optical signal IN is received from an input port of the VOA chip510 by the ball lens BL and coupled to a first or proximate portion of awaveguide 514. The waveguide 514 propagates the optical signal viaseveral tight bends or folds (illustratively three) in the waveguide toa distal end of the waveguide, which is in communication with an outputport via the ball lens BL. The ball lens BL propagates an optical outputsignal OUT to the output port and any subsequent optical transmissionmedium.

A portion of the waveguide 514 is disposed between a p-type region 516+and an n-type region 516− of a VOA 516. A portion of the waveguide 514optically communicates with a tap 511, which diverts a portion of theoptical signal to a detector 513. The detector 513 generates a powersignal P indicative of the power of the optical signal. The power signalP is provided to the VOA controller 520, which responsively generates acontrol signal C used to control a bias circuit 518. The bias circuit518, in response to the control signal C, provides a bias control signalBC which is coupled to the VOA 516. The VOA 516, in response to the biascontrol signal BC, injects a proportionate number of carriers into thewaveguide 514 to effect thereby a proportionate or controlledattenuation of the optical signal conveyed via the waveguide 514.

Thus, the optical waveguide apparatus of FIG. 5 provides a compact,low-cost VOA chip in which a single ball lens conveys input and outputoptical signal to a highly confined optical waveguide 514. The opticalsignal carried by the highly confined optical waveguide 514 is sampledand its power level detected such that appropriate adjustments in powerlevel are made by controllably attenuating the optical signal using theVOA 516. In this manner, a VOA chip providing optical measurement andattenuation functions are provided on a single substrate.

FIG. 6 depicts an avalanche photodiode (APD) receiver apparatusaccording to one embodiment. Specifically, an APD receiver 600 comprisesan APD receiver chip 610 and a VOA controller 620. The APD receiver chip610 comprises a substrate 612 including a trench T adapted to receive aball lens BL. An input optical signal IN is received from an input portof the VOA chip 610 by the ball lens BL and coupled to a first orproximate portion of a waveguide 614. The waveguide 614 propagates theoptical signal back to an avalanche photodiode (APD) 615 via severaltight bends or folds (illustratively two) in the waveguide 614. The APDprovides an electrical output signal OUT in response to the opticalsignal provided via the waveguide 614. The output signal OUT is usefulwithin the context of an optical receiver (i.e., the receiver apparatusof FIG. 6 finds particular utility within the context of a front end ofa receiver).

A portion of the waveguide 614 is disposed between a p-type region 616+and an n-type region 616− of a VOA 616. A portion of the waveguide 614optically communicates with a tap 611, which diverts a portion of theoptical signal to a detector 613. The detector 613 generates a powersignal P indicative of the power of the optical signal. The power signalP is provided to the VOA controller 620, which responsively generates acontrol signal C used to control a bias circuit 618. The bias circuit618, in response to the control signal C, provides a bias control signalBC which is coupled to the VOA 616. The VOA 616, in response to the biascontrol signal BC, injects a proportionate number of carriers into thewaveguide 614 to effect thereby a proportionate or controlledattenuation of the optical signal conveyed via the waveguide 614. Thus,by controlling the VOA 616 the optical power of the signal provided tothe APD 615 is controlled such that the APD is not damaged by an opticalsignal having the power level above an APD damage threshold level.

The inventors note that typical commercial receivers based on APDsrequire an external VOA outside and separate from the receiver packageto prevent high input optical powers from damaging the sensitive APD.This requires an extra optical package, since the VOA is amicro-electromechanical device and the APD is, illustratively, an IndiumPhosphate (InP) device and both are made in different materials usuallyby different vendors. The APD receiver apparatus of FIG. 6 combines aVOA, illustratively a polarization independent VOA, with a silicon APDsuch that the VOA and ADP functions may be monolithically integrated tothereby reduce cost. Furthermore since the silicon APD does not have anexposed junction, no hermetic package is required which will furtherreduce costs. In various embodiments, the wafer or substrate maycomprise a silicon on insulator substrate, an Indium-Phosphatesubstrate, or other substrate.

FIG. 7 depicts an optical waveguide apparatus according to anotherembodiment. Specifically, an optical waveguide apparatus 700 comprises aVOA chip 710 and a VOA controller 720. The VOA chip 710 comprises asubstrate 712 including a trench T adapted to receive a ball lens BL.

An input optical signal IN is received by the ball lens BL from an inputport of the VOA chip 710 and coupled to a first or proximate portion ofa waveguide 714. The waveguide is adapted to form a Mach-Zehnderstructure on the VOA chip 710. Specifically, the waveguide 714 is splitinto two Mach-Zehnder arms denoted as first arm 714A and second arm 714Bat point 719 by, illustratively, an optical splitter. Each arm 714A and714B of the Mach-Zehnder structure passes through (e.g., has disposedabout it) structure implementing respective VOAs 716A and 716B, whichare capable of imparting attenuation to the respective optical signalpassing through the waveguide arm. Specifically, each arm 714A, 714B isdisposed between a respective p-type region 716+ and a respective n-typeregion 716− of a respective VOA 716, which regions are controllablysubjected to bias currents.

After the VOAs 716, the two waveguide arms 714A and 714B are coupledtogether at point 719B and the waveguide 714 propagates optical signalto a distal end of the waveguide, which is in communication with anoutput port via the ball lens BL. The ball lens BL propagates an opticaloutput signal OUT to the output port and any subsequent opticaltransmission medium. As with the embodiments discussed above, thewaveguide 714 propagates optical signal via several tight bends or folds(two, three or more) in the waveguide to provide thereby the opticaloutput signal OUT.

A portion of the optical signal traversing the waveguide 714 is divertedby tap 711 and provided to a detector 713. The detector 713 generates apower signal P indicative of the power of the optical signal traversingthe waveguide 714. The power signal P is provided to the VOA controller720, which responsively generates a control signal C used to control abias circuit 718. The bias circuit 718, in response to the controlsignal C, provides respective bias control signals BCA and BCB to thefirst 716A and second 716B VOAs. The first 716A and second 716B VOAs, inresponse to the respective bias control signals BCA and BCB, inject aproportionate number of carriers into the respective waveguide portions714A and 714B to effect thereby a proportionate or controlledattenuation of the optical signal conveyed via the respective waveguideportions 714A and 714B.

The two VOAs 716A and 716B may be independently operated in forward biasand/or reverse bias mode. In a forward bias mode, the VOA 716 injectscarriers into the P-I-N junction of the intrinsic region of its portionof the split waveguide 714, which thereby increases/initiatesattenuation of the optical signal passing there through. In a reversebias mode, the VOA 716 ceases to inject carriers into the P-I-N junctionof the intrinsic region of its portion of the split waveguide 714, whichthereby reduces/eliminates attenuation of the optical signal passingthere through.

It is noted that in the reverse bias mode, carriers are moved across theP-I-N junction without generating significant currents. However, bymoving the carriers across the P-I-N junction in reverse bias mode, theoverlap between the optical mode and the location of the carriers ischanged. This overlap parameter is controllably increased or decreasedby changing the reverse bias current to provide thereby a change inphase of the optical signal traversing an arm. By reverse biasing bothP-I-N junctions (i.e., controlling a reverse bias level of both VOA 716Aand 716B), the phase of the optical signal through the two arms ischanged without changing the resulting optical signal intensity at theoutput port.

The operation of the embodiments of FIG. 7 are different than theoperation of the embodiments discussed above with respect to FIGS. 4-6.Specifically, in the FIGS. 4-6 embodiments the light intensity of awaveguide is directly modulated by injecting large carrier density intothe intrinsic regions of the P-I-N junction. By contrast, the FIG. 7embodiments utilize two VOAs within the context of a Mach-Zehnderstructure to, in a reverse bias mode of operation, indirectly modulateintensity by modulating phase and optical mode.

The carrier injection process discussed above with respect to the VOAsof FIGS. 4-6 is relatively slow (e.g., ˜500 MHz to ˜1 GHz) compared tothe reverse bias process discussed herein with respect to the VOAs ofFIG. 7 (e.g., ˜10 GHz to ˜20 GHz). However, with a faster VOA comes areduction in extinction ratio (i.e., attenuation ratio). Thus, inapplications where speed is more critical than extinction ratio orattenuation level, the VOA discussed herein with respect to FIG. 7 isappropriate. Similarly, in applications where large extinction ratio orattenuation level is more critical than speed, the VOA discussed hereinwith respect to FIGS. 4-6 are appropriate.

Various embodiments described herein with respect to the various figureshave been depicted as using an on-chip bias control circuit (e.g., 318,418, 518, 618 and 718). While an internal (on-chip) bias circuit islikely to be a more efficient design, the bias circuit may also comprisean external (off-chip) circuit.

Various embodiments described herein with respect to the various figureshave been depicted as using an external VOA controller (e.g., 320, 420,520, 620 and 720) which provides a control signal C to an on-chip oroff-chip bias circuit in response to optical power indicative signal. Itwill be appreciated that the VOA controllers described herein may bedigital devices such as digital signal processors (DSPs), fieldprogrammable gate arrays (FPGAs), microprocessors or other computingelements/devices, analog processing elements and the like. Moreover,while depicted as being off-chip VOA controllers, it will be appreciatedby those skilled in the art that the VOA controller function may beimplemented on a VOA chip or APD receiver chip. Alternatively, such achip may include upon the substrate a simple feedback loop adapted tokeep optical power level below a threshold level, such as requiredwithin the context of an APD receiver.

A VOA controller may be preprogrammed or programmed via data receivedfrom an external computer or other device (not shown). Such programmingmay be used for both digital and analog VOA controllers. Generallyspeaking, in operation the VOA controller implements a bias control loopthat adapted to maintain a constant optical signal power level at thewaveguide output or chip output port.

It should be noted that, while only depicted with respect to the VOAcontroller 720 of FIG. 7, the external programming port (receiving thesignal PROGRAM DATA) and its related functionality is optionallyprovided with any of the VOA controllers depicted in the other figures.Further, while only depicted with respect to the VOA controller 720 ofFIG. 7, the external control signal port (receiving the signal EXTERNALCONTROL SIGNAL) and its related functionality is optionally providedwith any of the VOA controllers depicted in the other FIGS.

In one embodiment, the VOA controller includes a lookup table in whichthe level of a received signal indicative of waveguide optical power ismapped to an optical power profile (linear or nonlinear). In response toindication of a waveguide optical power level that is too high, too low,outside a desired range, or otherwise beyond a threshold level, the VOAcontroller adapts the signal provided to the bias control circuit suchthat the attenuation imparted by the VOA to the waveguide tends to bringthe waveguide optical power to an appropriate or desired level. It isalso noted that the control function is useful within the context ofmaintenance of optical networks where optical signal power may bereduced for safety reasons to allow workers to access certain systemcomponents without sustaining eye damage.

Generally speaking, a digital VOA controller may be implemented as ageneral purpose computer including memory, input/output means and aprocessor. Specifically, the various control functions are optionallyimplemented by a VOA controller configured as a computer program productwherein computer instructions, when processed by the computer, adapt theoperation of the computer such that the methods and/or techniquesdescribed herein are invoked or otherwise provided. Instructions forinvoking methods described herein may be stored in fixed or removablemedia and/or stored within a memory within a computing device operatingaccording to the instructions. Also, at least portions of theinstructions may be transmitted via a data stream in a broadcast orother signal bearing medium.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. Apparatus, comprising: a substrate, havingdisposed thereon a waveguide; an epi-silicon region disposed upon thewaveguide and sized to reduce a polarization sensitivity characteristicof the waveguide; and a variable optical attenuator (VOA) comprising ap-type region and an n-type region, the VOA disposed about the waveguideand responsive to a bias current to controllably inject carriers intothe waveguide to attenuate thereby optical signal propagating throughthe waveguide.
 2. The apparatus of claim 1, wherein the substratecomprises one of a silicon-on-insulator wafer and an Indium-Phosphatewafer.
 3. The apparatus of claim 1, wherein the waveguide is highlyconfined.
 4. The apparatus of claim 1, wherein the geometry of theepi-silicon region is selected to enable a polarization insensitivity tothe apparatus, whereby the apparatus supports both TE and TMpolarizations.
 5. The apparatus of claim 1, wherein the epi-siliconregion is formed as a layer separated from the waveguide by a transitionregion.
 6. The apparatus of claim 1, wherein the epi-silicon region isformed as part of a P-I-N injection region including the VOA.
 7. Theapparatus of claim 1, further comprising an input ball lens disposedupon the substrate and adapted to communicate optical signal toward aproximate end of the waveguide, and an output ball lens disposed uponthe substrate and adapted to communicate optical signal from a distalend of the waveguide.
 8. The apparatus of claim 7, wherein the substrateincludes trenched portions adapted to receive the ball lenses.
 9. Theapparatus of claim 1, further comprising a single ball lens disposedupon the substrate and adapted to communicate input optical signaltoward a proximate end of the waveguide, and to receive output opticalsignal from a distal end of the waveguide.
 10. The apparatus of claim 2,wherein the waveguide is highly confined.
 11. The apparatus of claim 1,wherein the substrate includes a trench adapted to receive a ball lens,the ball lens adapted to couple an input optical signal to a proximateend of the waveguide, and to communicate an output optical signal from adistal end of the waveguide toward an external optical transmissionmedium.
 12. The apparatus of claim 9, wherein the ball lens receives theinput optical signal from a first optical propagation medium andcommunicates the output optical signal toward a second opticalpropagation medium.
 13. The apparatus of claim 9, wherein the first andsecond optical propagation media comprise any of optical fibres, siliconnanowires, lightpipes, waveguides.
 14. The apparatus of claim 1, furthercomprising: a bias circuit adapted to provide said bias current inresponse to a bias control signal; a tap for sampling optical signalpassing through the waveguide; a detector for generating a powerrepresentative signal in response to sampled optical signal receivedfrom the tap; and a controller for generating the bias control signal inresponse to the power representative signal, the controller adapting thebias control signal in response to waveguide optical signal power levelsoutside of a desired range.
 15. The apparatus of claim 14, wherein thebias circuit, tap and detector are disposed upon the substrate.
 16. Theapparatus of claim 9, further comprising: a bias circuit adapted toprovide said bias current in response to a bias control signal; a tapfor sampling optical signal passing through the waveguide; a detectorfor generating a power representative signal in response to sampledoptical signal received from the tap; and a controller for generatingthe bias control signal in response to the power representative signal,the controller adapting the bias control signal in response to waveguideoptical signal power levels outside of a desired range.
 17. Theapparatus of claim 13, wherein the substrate further includes anavalanche photodiode (APD) positioned between the a distal end of thewaveguide and an output port, wherein the bias current is adapted toconstrain optical power levels in the waveguide to below a thresholdlevel.
 18. Apparatus, comprising: a substrate, having disposed thereon awaveguide; an epi-silicon region disposed upon the waveguide; a variableoptical attenuator (VOA) comprising a p-type region and an n-typeregion, the VOA disposed about the waveguide and responsive to a biascurrent to controllably inject carriers into the waveguide to attenuatethereby optical signal propagating through the waveguide; and a singleball lens disposed upon the substrate and adapted to communicate inputoptical signal toward a proximate end of the waveguide, and to receiveoutput optical signal from a distal end of the waveguide.
 19. Apparatus,comprising: a substrate, having disposed thereon a waveguide; anepi-silicon region disposed upon the waveguide; and a variable opticalattenuator (VOA) comprising a p-type region and an n-type region, theVOA disposed about the waveguide and responsive to a bias current tocontrollably inject carriers into the waveguide to attenuate therebyoptical signal propagating through the waveguide; wherein the substrateincludes a trench adapted to receive a ball lens, the ball lens adaptedto couple an input optical signal to a proximate end of the waveguide,and to communicate an output optical signal from a distal end of thewaveguide toward an external optical transmission medium.